Hmm, this is wierd. When I try and run DoS the command box will pop up and immedietly close. Could I have a corrupted dos applet?
Is it just the standard command line, or are you running a program in DOS? I know when I run Perl programs standalone on my computer (which run in a DOS command line), the program will often run, and then close the window right away. If it's just a standard command line you are running, try going to Start -> Run and type in command and hit enter. If that doesn't work ... *shrugs* you've got a problem, might want to do a virusscan and/or reinstall Windows.
I ran into the same problem the other day. My problem was I was running a program on a drive that the command prompt was also using. You may want to close all programs that could possibly be conflicting. Before you reinstall the OS, reboot and close all unnessary tasks in the task manager and then try the dos prompt again to make sure it was just a conflict problem.